1. Field of the Invention
The present invention relates to liquid crystal cells, more particularly those using a smectic phase liquid crystal for the purpose of displaying images or patterns. It relates more particularly to devices using two addressing modes: optical addressing by means of a laser beam and matrix addressing by means of heating electrodes.
2. Description of the Prior Art
The use is known in the prior art of the thermo-optical effect in smectic phase liquid crystals for storing and displaying information. This effect has in particular been used in a laser addressing optical valve for constructing a high resolution projection display. A screen comprising 2000.times.2000 resolution dots can be easily formed. The typical dimension of a dot is then 20 microns. To erase the recorded image, it is possible either to scan the cell with the light beam at full strength and then slowly cool the layer, or to subject the layer to an alternating field with or without beam scanning. The problem with this type of display is however, for some applications, its writing speed: of the order of some 10.sup.4 dots per second.
Use is also known in the prior art of matrix addressing for displaying information at television rate with electrode networks whose pitches are typically between 20 and 100 microns. This method however requires, for smectic phase mesomorphic materials, that the liquid crystal layer be heated, for example by Joule effect, by causing an electric current to flow in strip shaped resistive elements.
The present invention proposes combining these two types of display to solve the problem often posed of a display with high resolution and slow to moderate renewal rate (a few seconds for example) with visualization of more elementary information at a rapid rate. This will generally be the display of a few alphanumeric characters in one of the corners of the screen (advance of a counter, of an index, of the time etc . . . ). The device proposed is particularly intended to be observed by reflection and projection on a screen. The mesomorphic material may also contain dispersed particles having a modulating effect on the reading radiation of the cell.